1. Field of the Invention
The present invention relates to an electrostatic chuck, specifically, for holding a semiconductor wafer (hereinafter, called wafer) or a liquid crystal glass in an etching step for minutely processing, a depositing step for forming a thin film, or a exposing step for exposing a photoresist film, on the wafer or the liquid crystal glass in a semiconductor or liquid crystal manufacturing process.
2. Description of the Related Art
Conventionally, an electrostatic chuck is used for holding a wafer using electrostatic force in an etching step for minutely processing, a depositing step for forming a thin film, or a exposing step for exposing a photoresist film, on the wafer in a semiconductor manufacturing process.
This electrostatic chuck, as shown in FIG. 5, includes a pair of chucking electrodes 53 on the top face of a ceramic base 54, and power supply terminals 58, where an insulating film 52 is formed over the chucking electrodes 53. The top surface of the insulating film 52 serves as a placing surface 52a for placing a wafer.
The electrostatic chuck 51 is an object holding apparatus utilizing the Coulomb force of static electricity. When the insulating film 52 with a dielectric constant ε and a thickness r is formed and the wafer is placed on the placing surface 52a and then voltage V is applied between the chucking electrodes 53, half V/2 of the voltage is applied between the wafer W and each of the chucking electrodes 53. The half voltage causes the electrostatic force for pulling the wafer W. The chucking force F per unit area of this chucking force is calculated by the following formula:F=(ε/2)*(V2/4r2)
The chucking force F that is an electrostatic force for holding an object increases as the thickness r of the insulating film 52 becomes smaller and the voltage V becomes higher. The higher the voltage V becomes, the more the chucking force F increases. But if it is too much, insulation of the insulating film 52 might be broke down. In addition, in case a void, such as pinhole, exists in the insulating film 52, the insulation might be broke down. Therefore, the surface of the insulating film 52 for holding an object requires smoothness and lack of pinhole.
By the way, a typical electrostatic chuck, as disclosed in the document 1 (JP-A-59-92782 (1984)), includes a metal, such as aluminum, for the electrode and a glass or organic film, such as bakelite, acrylic or epoxy, for the insulating film covering the electrode. However, these insulating film have problems in view of heat resisting properties, wear resistance, chemical resistance, etc., as well as cleanliness because abrasive powder which is generated in operation is likely to stick to a semiconductor wafer with bad influence.
Additionally, another electrostatic chuck, as shown in FIG. 3, which includes a ceramic film formed by spray coating for the insulating film 25 is disclosed in the document 2 (JP-A-58-123381 (1983)). This insulating film has a number of pinholes with a problem of withstand voltage.
Moreover, the document 3 (JP-A-4-49879 (1992)) discloses a method for forming chucking electrodes on the principal plane of a ceramic base, and then forming an insulating film with a thickness of several micrometers over the principal plane of the ceramic base using sputtering, ion plating or vacuum deposition.
For the requirement of an electrostatic chuck used in a etching process, it can be used in a range of −20 to 200 degree-C because the process temperature is changed depending on plasma-resistance in halogen corrosive gas, such as process gas or cleaning gas, and the species of film to be etched.
Processes requiring the plasma-resistance are increasingly demanded, since minute processing is increasingly developed for expansion of memory capacity of VLSI. Especially, halogen corrosive gas, such as chlorine gas, fluorine gas, is frequently used for etching gas or cleaning gas. In a cleaning step, wafer-less cleaning method in which cleaning is performed with no dummy wafer on a wafer placing face is studied. The method might strongly require the plasma-resistance of the wafer placing face.
The electrostatic chuck might require a wide range of operation temperature and durability, depending on the species of films on a wafer to be etched. Disclosed are an electrostatic chuck which includes a conductive base of aluminum alloy and a spray coating film of alumina on the surface, and another electrostatic chuck which includes a conductive base of aluminum alloy and an anodized film of aluminum for an insulating film, to complete the plasma-resistance. But these have a problem of cracking due to the difference of the thermal expansion between the aluminum base and the insulating film when temperature is rising. For the countermeasure, the document 4 (JP-A-11-265930 (1999)) discloses an electrostatic chuck which includes a spray coating film 25 of alumina for the insulating film in consideration of a coefficient of thermal expansion of the conductive base 23 made of ceramics and metal, to prevent cracking even in a wide range of operation temperature.
The document 5 (JP-A-8-288376 (1996)) discloses an electrostatic chuck which includes a conductive base of aluminum alloy, an anodized film of aluminum on the surface, and an amorphous aluminum oxide with a thickness of 0.1 to 10 μm formed thereon having excellent plasma-resistance.
The document 6 (JP-A-4-287344 (1992)) discloses an electrostatic chuck which has chucking electrodes inside of ceramics, which is integrated with a conductive base equipped with a cooling function using silicone adhesives.
The insulating film in the electrostatic chuck disclosed in the documents 3 and 5 is formed using sputtering or CVD, the thickness of which is limited to a few micrometers or less, therefore, causing a possibility that the insulation of the insulating film is broken down when voltage is applied to the chucking electrodes.
In the document 5, as shown in FIG. 4, an anodized film 26 of aluminum is formed on the surface of a base 24 of aluminum alloy and an amorphous aluminum oxide layer 22 having excellent plasma-resistance is formed thereon with a thickness of 0.1 to 10 μm. But in such a protective film with a thickness of about 10 μm, pinholes which are created at a film forming step cannot be filled in, thereby eroding the underlayer. In addition, such a film with a thickness of 0.1 to 10 μm is easily eroded under a hard plasma condition, so lacking for practicality. This film also has a problem of flaking off due to an internal stress when forming the film having a thickness of 10 μm or more.
Further, since the base 24 of aluminum alloy is used for the conductive base, the film is cracking at a temperature of 100 degree-C. or higher because of the different coefficients of thermal expansion of the anodized film 26 of aluminum and the amorphous aluminum oxide layer 22 formed thereon.
Furthermore, in case the volume resistivity of the upper amorphous aluminum oxide film 22 is larger than that of the lower anodized film of aluminum, the voltage between the conductive base 24 of the electrostatic chuck 21 and the wafer is weighted toward the amorphous aluminum oxide film 22, resulting in breakdown of the insulation of the amorphous aluminum oxide film 22.
Since the amorphous aluminum oxide film is different in volume resistivity from the anodized film of aluminum, the chucking force cannot rise up immediately and it takes time to become a certain level when voltage is applied. When the applied voltage is turn off, the chucking force cannot be zero immediately and the residual chucking force take places. Thus these response of chucking and release characteristics is degraded and it takes excessive time to attach or detach the wafer, resulting in disadvantage for the process control.
In an electrostatic chuck disclosed in the document 6 (JP-A-4-287344 (1992)), silicone adhesives has a problem that a layer of silicone adhesives is eroded by process gas in an etching equipment.
The insulating film 25 which is formed of a spray coating film of alumina, as disclosed in the patent documents 2 and 4, has a number of voids, which are sealed later using organic silicon or inorganic silicon. But the sealed portion of silicon is easy to be etched by plasma, thereby lowering the withstand voltage. The electrostatic chuck would not working in a short period of time.